1. Field of the Invention
The present invention relates to a method of manufacturing spatial light modulator and electronic device employing it.
2. Description of the Related Art
This type of spatial light modulator is disclosed, for example Japanese Patent Application Laid-Open Nos. 4-230722, 5-188308, and 5-196880. An improved form of these devices is also described in the March 1994 issue of xe2x80x9cNikkei Microdevicexe2x80x9d as a xe2x80x9cDMDxe2x80x9d (Digital Micromirror Device).
This DMD has, as shown in FIG. 22, a three-layer construction comprising an upper layer 800, an intermediate layer 810, and a lower layer 830.
The upper layer 800 comprises a mirror 802 and a mirror support post 804 joined to the center of the lower surface of the mirror 802. In connection with the fabrication process of the mirror 802, in a position opposite to the mirror support post 804 is formed a depression 806.
The intermediate layer 810 has a mirror support plate 812 which is coupled to the mirror support post 804, and which is supported at opposite ends by hinges 814 so as to be able to be driven in an inclining manner. To provide the space for this mirror support plate 812 for driving in an inclining manner, the hinges 814 have on their lower sides hinge support posts 816.
The intermediate layer 810 further is provided with first and second address electrodes 818 and 820 on opposing sides of the hinges 814, each supported by electrode supporting posts 826. Furthermore, outside this are provided a first mirror contact electrode 822 and second mirror contact electrode 824, each supported by electrode supporting posts 826.
The lower layer 830 comprises four electrodes 832a to 832d coupled to the electrode supporting posts 826 of the first and second address electrodes 818 and 820, and a common electrode 834 coupled to the first and second mirror contact electrodes 822 and 824.
This DMD, as shown in FIG. 23, has a bias voltage Va applied to the mirror 802 and the first and second mirror contact electrodes 822 and 824. Then when for example a negative voltage is applied to the first address electrode 818, and a positive voltage is applied to the second address electrode 820, a Coulomb force acts between the mirror 802 and the first address electrode 818, and the mirror 802 is driven to an inclined position as shown by the dot-dash line in FIG. 23. By reversing the polarity of the voltage applied to the first and second address electrodes 818 and 820, an inclined position as shown by the dot-dot-dash line in FIG. 23 can be established.
The inclined position of the mirror 802 shown by a dot-dash line in FIG. 23 is taken to be the xe2x80x9cONxe2x80x9d position in which light is reflected toward a certain position, and the inclined position shown by a dot-dot-dash line is taken to be the xe2x80x9cOFFxe2x80x9d position in which light is reflected in a different direction. By varying the time between switches, a 256-gradation display can be obtained.
The DMD shown in FIG. 22 is hypothetically manufacturable by a fabrication process as shown in FIGS. 24A to 24H and FIGS. 25A to 25F. FIGS. 24A to 24H show the steps in the formation of the intermediate layer 810 on an already formed lower layer 830, and FIGS. 25A to 25F show the steps in the formation of the upper layer 800 on the intermediate layer 810, and the formation of the interlayer spaces.
As shown in FIG. 24A, a substrate 840 on which an SRAM (static random access memory) is formed as the lower layer 830 is provided. Next, as shown in FIG. 24B, a resist 842 is coated on this substrate 840, and in the stage shown in FIG. 24C a pattern corresponding to the hinge support posts 816 and electrode supporting posts 826 is formed.
As shown in FIG. 24D, an aluminum (Al) film is formed by vapor deposition over the surface of the resist 842 and trench portion, and then further as shown in FIG. 24E an aluminum oxide film 846 is formed over the surface.
Further after vapor deposition of an aluminum film 848 as shown in FIG. 24F, as shown in FIG. 24G a resist 850 is applied in a pattern. Thereafter, as shown in FIG. 24H, the aluminum film 848 is etched, whereby mirror support plate 812, hinges 814, and hinge support posts 816 are formed.
By the process shown in FIGS. 25A to 25F, the upper layer 800 shown in FIG. 22 is formed. For this purpose, as shown in FIG. 25A a resist 852 is applied thickly, and is formed in a pattern as shown in FIG. 25B. Further, an aluminum film 854 is formed by vapor deposition, and after an aluminum oxide film 856 is formed over a part of the surface thereof, the extremities of the aluminum film 854 are removed by etching, whereby the mirror 802 and mirror support post 804 are formed. (See FIGS. 25C to 25E.)
Finally, as shown in FIG. 25F, by removing the resist 842 and 852, a space between the upper layer 800 and intermediate layer 810 is formed, and moreover a space between the intermediate layer 810 and lower layer 830 is formed.
However, in the above process, there is the problem that the DMD cannot be obtained with a high yield. One reason for this is that the factor determining the angle of inclination of the mirror 802, that is, the distance between the lower surface of the mirror 802 and the mirror contact electrodes 822 and 824 depends on the thickness of the resist 852 in the resist step shown in FIG. 25A.
In general, such a resist is formed by the spin coating method, and while it is difficult in itself to improve the uniformity of a resist layer thickness, when the spin coating method is used it is extremely difficult to make the resist 852 of a uniform thickness.
Moreover, in the conventional spin coating method, the larger the surface area of the wafer, the more difficult it is to ensure uniformity within the area of the resist film, and further to make the thickness of the resist film constant is for a large diameter semiconductor wafer almost impossible. Thus, it is difficult to form a plurality of devices simultaneously from a single semiconductor wafer, and the throughput is reduced.
In addition to the above problems, a further one is that in the stage of removing the resist shown in FIG. 25F, it is difficult to completely remove the resist from the furthest recesses of the underside of the mirror 802 and hinges 814. If foreign objects are thus left behind, the mirror 802 and address electrodes 818 and 820 may short-circuit, or the inclination of the mirror may be obstructed, or the mirror contact electrodes 822 and 824 and address electrodes 818 and 820 may short-circuit.
Another problem with the above described construction of a DMD is that the depression 806 is formed in the center region of the mirror 802. In the aluminum vapor deposition step of FIG. 25C, when aluminum is vapor deposited in the trench portion, the position opposing this trench is inevitably concave, and the forming of the depression 806 cannot be prevented.
In this three-layer DMD, since the hinges 814 are not in the same plane as the mirror 802, the exposed surface area of the mirror 802 is increased, and the benefit is obtained of an increased light utilization ratio.
However, since the depression 806 is formed in the center of the large area mirror 802, with this depression 806 in the line of a powerful beam of light, the light utilization ratio is actually reduced by the diffuse reflection. Alternatively, the diffusely reflected light may be input as information pertaining to another pixel, resulting in the problem of reduced image quality. Moreover, even if the side walls of the depression 806 are processed so as to be vertical, the area which is optically effective is reduced.
A further problem is that the above described spatial light modulator is formed on a substrate 840 on which an SRAM is formed, and the overall yield is the product of the yield of the SRAM and the yield of the spatial light modulator, which is thus considerably low.
Another prior art is the spatial light modulator described in Petersen, xe2x80x9cSilicon as a Mechanical Materialxe2x80x94Proceedings of the IEEE, Vol. 70, No. 5, May 1982, in FIGS. 39, 40 and 41 on pages 442 and 448. In order to fabricate this, a silicon substrate which has been cut and ground on both sides is used, and a micromirror is formed on this silicon substrate by photolithography and etching processes. The silicon substrate on which this micromirror is formed and a glass plate on which a metal electrode film is formed are bonded by the anode bonding method, and a spatial light modulator thus manufactured.
By this method, however, in order to cut and grind the silicon substrate on both sides, and thus determine the substrate thickness, it is not possible to obtain a thickness less than 200 mm. This is because grinding to a thickness less than this leads to breakage of the silicon substrate. The thickness of the micromirror is therefore at least 200 mm, and the inertial moment due to this heavy mass is thus great, making rapid response and high resolution display impossible.
It is the object of the present invention to provide a spatial light modulator equipped with micromirrors which can be fabricated with a high yield, a method for manufacturing the same, and an electronic device employing the spatial light modulator.
Another object of the present invention is to provide a spatial light modulator equipped with micromirrors which allows accurate control of spatial light modulation without the generation of diffuse reflection on the surface of the micromirrors, a method for manufacturing the same, and an electronic device employing the spatial light modulator.
Yet a further object of the present invention is to provide a spatial light modulator for which the yield is high, and for which a moving-picture gradation display is easy, a method for manufacturing the same, and an electronic device employing the spatial light modulator.
The method of the present invention pertains to fabricating a spatial light modulator having micromirrors, by bonding together a conductive silicon mirror substrate and an electrode substrate. The conductive silicon mirror substrate has a plurality of micromirrors arranged in one of a line and in matrix and a torsion bar coupling the micromirrors in one direction, and a reflective layer is formed at least on one surface of the micromirrors.
The electrode substrate has a depression in a central region, a rim around the periphery thereof, a set of electrodes having conducting layers disposed within the depression in positions corresponding to the micromirrors, and driving the micromirrors in an inclining manner by means of a Coulomb force, and pillars projecting from the depression in positions corresponding to the interval between two of the micromirrors adjacent in the one direction.
In the step of bonding together the conductive silicon mirror substrate and the electrode substrate, at least intermediate portions of the torsion bar on the silicon mirror substrate are opposite to the pillars of the electrode substrate.
In this way, if the depression in the glass electrode substrate is previously formed with a depth of high accuracy, the deflection angle of the micromirrors can be determined accurately from lot to lot. Furthermore, the reflective layer formed on the surface of the micromirrors can be made uniform, and a surface with no diffuse reflection can be formed.
In particular, when for example the electrode substrate is employed a glass electrode substrate including an alkali metal such as sodium, the substrates can be bonded using anode bonding. This means that no adhesive layer is required between the substrates, and thus the deflection angle of the micromirrors can be determined accurately from lot to lot.
It should be noted that the bonding method is not, however, restricted to anode bonding, and direct bonding or diffusion bonding can also be used, and more detailed description of the bonding method is given below. Furthermore, if heat is applied in the bonding process, the material of the electrode substrate should preferably be a material with a coefficient of thermal expansion close to that of silicon.
On the silicon mirror substrate, a frame portion to which both ends of the torsion bar are coupled may be formed. In this case, the frame portion and both ends of the torsion bar are bonded to the rim of the electrode substrate. In addition, after this bonding, a step of cutting away both ends of the torsion bar from the frame portion.
By this means, the mutual positional relationship of a plurality of torsion bars is maintained by the frame, and therefore without precisely positioning each torsion bar the mutual positional relationship between them can be maintained during bonding to the electrode substrate.
The process of step of fabricating the silicon mirror substrate may comprise the steps of:
doping a silicon substrate with impurities to form a doped layer;
patterning a first mask for forming a window on one surface of said silicon substrate and a second mask for forming said plurality of micromirrors and said at least one torsion bar on the other surface of said silicon substrate;
etching said silicon substrate until said doped layer is exposed using said first mask;
a step of etching said doped layer using said second mask;
removing said first and second masks and forming said plurality of micromirrors and said at least one torsion bar from said doped layer; and
forming said reflective layers on one surface of said micromirrors of said doped layer.
In another aspect of the present invention, before the silicon mirror substrate is completed, while in the form of a silicon substrate, it is bonded with the electrode substrate.
The electrode substrate has a depression in a central region, a rim around the periphery thereof, a set of electrodes having conducting layers disposed within the depression in positions corresponding to the micromirrors, and driving the micromirrors in an inclining manner by means of a Coulomb force, and pillars projecting from the depression in positions corresponding to the interval between two of the micromirrors adjacent in one direction.
Bonded to this is a silicon substrate on one surface of which is formed a doped layer doped with impurities. At this time, at least the pillars of the electrode substrate and the doped layer are opposite and bonded.
In this step, since the step is carried out before the micromirrors are formed, positioning for the bonding operation is simple.
Thereafter, the silicon substrate is etched to remove same, leaving the doped layer, and a reflective layer is formed on the surface of the doped layer.
Thereafter, the doped layer is etched. At this point a plurality of micromirrors are formed in positions opposite the set of electrodes. The torsion bar is formed coupling the micromirrors in one direction, bonded to the pillars at positions intermediate between two of the micromirrors adjacent in that direction.
During the patterning for this etching step, when the positional relationship with the set of electrodes already formed on the electrode substrate is considered, with the accuracy of a photolithography process, the micromirrors can be formed with high precision.
Using this method, the substrate positioning for bonding is easy, and moreover since the micromirrors and so forth can be fabricated after bonding, the method can be applied to high density layout of the micromirrors.
It should be noted that in the above method anode bonding can be adopted, or a frame portion can be formed on the silicon mirror substrate.
When the micromirrors are arranged in a high density layout, the electrode substrate may be formed of a transparent glass electrode substrate, and then the position of the pattern of the set of electrodes be observed from the side of the glass electrode substrate, and using this pattern position as a reference, the mask pattern alignment for the etching of the silicon electrode substrate carried out.
In the method inventions above, if the impurity concentration of the doped layer is at least 1xc3x971018 atm/cm3, then during the etching of the silicon substrate the doped layer can be used to function as an etching stop layer.
The method of fabricating the glass electrode substrate may include the steps of:
masking positions corresponding to said rim and said pillars and etching a glass substrate including an alkalimetal to form said depression of a predetermined depth; and
forming said sets of electrodes on the base of said depression. In this case the depth of the depressions which affects the deflection angle of the micromirrors, depends on the etching conditions.
The set of electrodes may be formed as a set of transparent electrodes of for example ITO (indium tin oxide), and before the bonding, there may be a step of inspecting the presence of foreign objects between the glass electrode substrate and the silicon mirror substrate from the side of the glass electrode substrate. If this inspection is carried out before the bonding, the yield is increased and when carried out after the bonding, the ingress of foreign objects which is a cause of defective products can be detected easily.
There may be an additional step of bonding a transparent cover plate on the silicon mirror substrate so as to cover the silicon mirror substrate and in a position non-interference with the micromirrors driven in an inclining manner.
By means of this transparent cover plate, the ingress of foreign objects which would impede the driving in an inclining manner of the micromirrors can be prevented, and the element protected.
The device of the present invention has a conductive silicon mirror substrate doped with impurities and an electrode substrate bonded integrally, wherein the silicon mirror substrate, comprises:
a plurality of micromirrors arranged in one of a line and matrix and having reflective layers formed on one surface; and
a torsion bar coupling said micromirrors in one direction;
at least one said electrode substrate comprises:
a depression in a central region thereof;
a rim around the periphery thereof;
sets of electrodes formed within said depression in positions corresponding to said micromirrors and driving said micromirrors in an inclining manner by means of a coulomb force; and
pillars projecting from said depression in positions corresponding to an interval between two of said micromirrors adjacent in said one direction; and wherein
at least intermediate portions of said at least one torsion bar on said silicon mirror substrate are opposite said pillars of said electrode substrate, and said silicon mirror substrate and said electrode substrate are bonded. This bonding may be carried out by for example direct bonding or eutectic bonding.
The entire surface of the reflective layer formed on the micromirrors is formed as a flat surface. It can therefore reflect impinging light with an angle of reflection equal to the angle of incidence.
The set of electrodes is preferably formed as a set of transparent electrodes of for example ITO (indium tin oxide). By looking through the glass electrode substrate, the ingress of foreign objects between the set of electrodes and the micromirrors, which would result in a defective product, can easily be detected.
Where the micromirrors are opposite the set of electrodes an insulating film may be formed so that in when foreign objects ingress between the micromirrors and the set of electrodes, the serious problem of a short-circuit can be avoided.
The surface of the set of electrodes where the electrodes are opposite the insulating film formed on the micromirrors, may further be formed to be rough. The contact area between the insulating film and the set of electrodes is reduced, and the micromirrors sticking to the set of electrodes caused by static charge on the insulating film can be prevented.
The surface roughness is preferably provided by forming on the surface of the set of electrodes projections of height at least 200 Angstroms. In this way adequate roughness can be assured to prevent sticking between the micromirrors and the set of electrodes. It should be noted that if the gap between the micromirrors and the set of electrodes when the micromirrors and set of electrodes are parallel is G, then the upper limit to the height of these projections should be not more than G/3. This assures the minimum deflection angle of the micromirrors required for functional reasons.
To prevent sticking of the micromirrors, an insulating projection may be formed on the insulating film and at a position displaced from the torsion bar.
As another method of preventing sticking of the micromirrors, an insulating stopper may be formed. The insulating stopper projects from the base of the depression of the glass electrode substrate to a height less than the height of the rim and the pillars, and abuts the micromirrors when driven in an inclining manner, in order to determine the deflection angle.
Using the spatial light modulator of the present invention, various electronic devices can be constructed.
For example, a projector can be constructed from a projection lamp, a spatial light modulator which reflects light emitted by the projection lamp modulated for each pixel by driving in an inclining manner each of a plurality of micromirrors arranged one per pixel, and a projection lens which projects an enlarged image of the light reflected from the spatial light modulator on a screen.
An electronic photography apparatus can be constructed from a photosensitive drum on which a latent image is to be formed, a spatial light modulator which reflects light sequentially, and emits reflected light modulated while scanning in one direction toward the photosensitive drum to form a latent image by driving in an inclining manner each of a plurality of micromirrors arranged in an array, a developing device developing the latent image formed on the photosensitive drum, and a transfer device transferring the image on the photosensitive drum to a recording medium.
Further, an optical switching device can be constructed from a plurality of induction coils capable of generating desirable induction voltages, a spatial light modulator, and a wiring pattern connecting the induction coils and a set of electrodes of the spatial light modulator, a plurality of the micromirrors are each driven in an inclining manner, and a desirable optical signal is generated by light reflected from the micromirrors based on the induction voltages generated by each of the induction coils.
In an exposure device which irradiates an exposure target with light from a light source through an interposed mask to expose the exposure target, a spatial light modulator may be provided to reflect the light from the light source from individual micromirrors, thus irradiating the exposure target with modulated light.
In this way, it is possible to record ID information such as a lot number using an exposure process on an exposure target such as a semiconductor wafer.
Another aspect of the spatial light modulator of the present invention, comprising:
a glass substrate on which at least one conductive torsion bar coupling a plurality of conductive micromirrors in one direction is supported by pillars, and on which a conductive frame portion fixing both ends of said at least one torsion bar is formed; and
a circuit substrate on which a plurality of pairs of electrodes opposite each of said micromirrors and a circuit element energizing said plurality of pairs of electrodes are formed;
and wherein said frame portion of said glass substrate and said circuit substrate are bonded.
In this way, the micromirrors and the circuit substrate can be fabricated separately, and foreign objects can also be inspected separately, as a result of which the yield can be increased. Moreover, the region in which the micromirrors are disposed is covered by the glass substrate, frame portion, and circuit substrate.
The micromirrors and torsion bar may be formed from silicon or a metal.
The method of fabricating the micromirrors of silicon, comprises:
(a) forming on a glass substrate a depression in a central region thereof, a rim surrounding said depression, and pillars formed to project from said depression;
(b) diffusing impurities into one surface of a silicon substrate to a predetermined depth;
(c) further diffusing impurities into a predetermined portion of said one surface of said silicon substrate to a predetermined depth to form an impurity diffusion surface;
(d) forming an optically reflective film on said impurity diffusion surface of said silicon substrate;
(e) bonding said impurity diffusion surface and said rim of said glass substrate, to form a silicon-glass bonded substrate;
(f) wet-etching said silicon-glass bonded substrate to make said silicon substrate into a thin film;
(g) dry-etching said silicon substrate of said thin film to form a plurality of micromirrors, a torsion bar coupling and supporting the same, and a frame portion fixing both ends of said torsion bar; and
(h) bonding to said frame portion of said silicon-glass bonded substrate a silicon circuit substrate provided with a plurality of pairs of electrodes for driving said plurality of micromirrors, and circuit elements applying a drive voltage to said electrodes.
On the other hand, the method of fabricating the micromirrors of a metal, comprises:
(a) forming a first resist pattern on a glass substrate to form pillars in a central region thereof and a first rim on the periphery thereof;
(b) forming a first metal film on said glass substrate and said first resist pattern;
(c) forming on said first metal film a second resist pattern to form micromirrors and a torsion bar;
(d) etching said first metal film using said second resist pattern;
(e) removing said second resist pattern;
(f) forming a third resist pattern in a region excluding a surface of said first rim;
(g) forming a second metal film on said first rim and said third resist pattern;
(h) forming a fourth resist pattern on said second metal film and in position opposite said first rim;
(i) etching said second metal film using said fourth resist pattern, and extending said first rim to form a second rim;
(j) removing said first, third, and fourth resist patterns; and
(k) bonding a silicon circuit substrate provided with circuit elements for driving said micromirrors and said second rim of said glass substrate.
In either of the methods, the circuit substrate and the glass substrate may be subjected to diffusion bonding or bonded using a conductive adhesive.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.